1. Field of the Invention
The invention relates to micro-mechanical rate-of-rotation sensors based on the Coriolis principle. In particular, the present invention pertains to sensors of the type that employ two plate-like oscillators, one above the other in parallel planes, each being capable of being stimulated to oscillate perpendicular to its plane by means of an electrostatic drive.
2. Description of the Prior Art
A type of rate-of-rotation sensor includes two plate-like oscillators, each formed in at least one wafer layer. The wafer layers are arranged one above the other in parallel planes and are stimulated to oscillate perpendicular to the planes in response to an electrostatic drive. Such a sensor is described in International patent application WO 96/38710 which is hereby incorporated by reference. A perspective view of the device presented in FIG. 6 (corresponding to FIG. 9 of the reference) clearly illustrates the prior art. Referring to the publication, such a rate-of-rotation sensor comprises two oscillators in alignment arranged one above another in layers. The upper oscillator 60 is visible in FIG. 6. Such oscillator 60 and a corresponding lower, mirror image, symmetrically arranged oscillator (not visible) is (in each case) articulated via a first spring 70 to an electrostatic plate-like drive 61, which, in turn, is connected via a second spring 69 to a plate-like support 62 through which the rate of rotation is read out. The whole arrangement, connected in a row, comprising the oscillator 60, the drive 61 and the support 62, is held in a frame 68 via a crossed-spring articulation 63, 63'. It can be seen from FIG. 6 that each oscillator element 60, 61, 62, including the associated frame 68, is formed in two layers (i.e. from a composite wafer) with the interposition of a thin insulation layer (not illustrated), of, for example, SiO2.
The upper two-layer frame 68 and the lower two-layer frame 68' thereby enclose the entire oscillator structure which is formed of four wafer layers. It is possible to supply different potentials via external connections 64 to 67, connected in one piece to the frame. Top and bottom wafers, provided with lead-throughs for electrostatic (capacitive) stimulation, signal read-out and resetting (in a closed-loop system) are not illustrated in FIG. 6; rather, reference is made in this regard to FIG. 2 of the cited reference. The advantage of such a two-layer oscillator structure as illustrated in FIG. 6 is, inter alia, that interference with the measured values resulting from reaction forces due to oscillator movements is avoided despite the fact that comparatively large oscillation amplitudes of the oscillator 60 and the mirror-image symmetrically arranged oscillator (not visible in FIG. 6) can be obtained with small capacitor drive gaps in the region of the drive 61. The rate of rotation is capacitively read out through area electrodes (not shown) at the upper side of the support 62 and at the lower side of the mirror-image lower support 62' (not visible in FIG. 6) employing corresponding mating electrodes on the top and bottom wafers (not shown). The illustrated crossed-spring articulation 63, 63' is advantageous in that rotational movements caused by Coriolis forces and the capacitance changes that follow are readily transmitted. On the other hand, horizontal and vertical oscillations are suppressed in this region.
In this known oscillator structure, electrostatic stimulation is made considerably simpler--as mentioned--due to the narrow drive gap in the region of the drive 61, in spite of the relatively large oscillation amplitudes. This can be implemented with comparatively low drive voltages (e.g., a few volts).
While simple oscillator systems are disadvantageous in that the reaction forces are dissipated into the mounting surface of the rate-of-rotation sensor, changes to the stiffness having reaction effects on the measuring system, with consequent zero point and scaling factor non-repeatability, a double oscillator has the advantage that its masses oscillate in opposite directions and thereby generate no net external reaction forces. However, it has been shown that oscillator frequencies differ as a result of oscillator mass and spring production tolerances. This results in problems for the drive electronics. The oscillators must be operated at an intermediate frequency, their amplitudes lowered in response to oscillator quality, with their phases incapable of being uniquely specified. Electronic solutions, by means of which both amplitudes and the mutual phases can be recorded, are complicated and susceptible to faults.